This invention relates to circuitry for driving an inductive load and the like and more particularly to such circuitry employing a pulse width modulation controller, especially for use in driving a motor or any other inductive load, especially when the load current is difficult to measure, for example a continuous current pulse width modulation power supply with isolated output.
Electric motors, for example a 3-phase brushed or brushless DC motor, can be driven by a pulse width modulation controller. In a pulse width modulated system, the switched current (of wave form of the type shown in FIG. 1) is often continuous, i.e. remains above zero, and the "fly-back" current (of wave form of the type shown in FIG. 2 and flowing during time T.sub.2) is generally flowing in the load but not in the supply.
In such a system in particular, measurement of the load current is not straightforward and requires the continual measurement of the amount of current flowing in each phase; this is a complex procedure.
The present invention is based on the realisation that the need to measure the load current in each phase can be obviated and that this current can be measured from other more readily determined parameters of the system.